Plague: Overview

 What is Plague

Plague, caused by a bacterium called Yersinia pestis, is transmitted from rodent to rodent by infected fleas.

Plague is characterized by periodic disease outbreaks in rodent populations, some of which have a high death rate. During these outbreaks, hungry infected fleas that have lost their normal hosts seek other sources of blood, thus increasing the increased risk to humans and other animals frequenting the area.

Source: CDC - Information on Plague Web-Page

 Overview

Many different animal species (mostly wild rodents) are natural reservoirs for Y pestis. Some animal populations are relatively resistant to the effects of Y pestis infection and serve as the enzootic reservoirs (see References: Dennis 1997). Other animal species are more susceptible to disease caused by Y pestis and serve as epizootic hosts.

Humans are incidental hosts for Y pestis and are not part of the natural life cycle of the organisms. Disease occurrence in humans is dependent on the frequency of infection in local rodent populations and the degree of contact between rodents and humans. Human outbreaks usually are preceded by epizootics with increased deaths in susceptible animal hosts.

Like humans, mammalian species other than rodents generally are incidental hosts for Y pestis. However, such animals also can serve as sources of human exposure (either through direct contact or through flea vectors).

The organisms most commonly are transmitted between animal reservoirs and to humans via bites of infected fleas. In order to survive in the flea midgut, Y pestis organisms require phospholipase D (PLD; formerly referred to as Yersinia murine toxin), which allows the organisms to be resistant to a cytotoxic digestion product of blood plasma in the flea gut. A recent study demonstrated that Y pestis acquired the PLD gene at some point in the past, which allowed transformation from a rather benign species of gut bacteria to a major global pathogen. Of the more than 1,500 flea species, about 30 are known to be vectors for Y pestis.

Source: CIDRAP - Plague: Current, Comprehensive Information on Pathogenesis, Microbiology, Epidemiology, Diagnosis, and Treatment